A communication device is a device provided with an appropriate signal receiving and transmitting arrangement for enabling communication with other parties. Typically a communication device is used for communication via a communication system for enabling the users thereof to receive and transmit communication such as speech and data. A communication system can be seen as a facility that enables communication sessions between two or more entities such as the communication devices and/or other nodes associated with the communication system. Subscribers or users to a communication system may be offered and provided numerous services via their communication devices. Non-limiting examples of these services include two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. User may also be provided broadcast or multicast content such as television programs. Examples of communication systems include fixed line communication systems, such as a public switched telephone network (PSTN), local area networks (LAN) and wireless communication systems, such as a public land mobile network (PLMN), satellite based systems and wireless local area networks (WLAN).
Communication with a communication device may be provided as point-to-point communications or point-to-multipoint communications. Point-to-point communications are also known as unicasting. Point-to-multipoint communications can be provided by means of multicasting or broadcasting. The distinction between these two forms or communication is rather vague but a view is that multicasting requires the user to join a group whereas broadcasting is provided without any interaction between the end-user and an entity responsible for sending the content. Regardless the type of the point-to-multipoint services, a common feature of them is that a shared stream of data can be simultaneously transmitted to a multiple of communication devices. That is, a number of recipients can receive the same data transfer flow.
A communication system may be provided with a point-to-multipoint service entity such as a multimedia broadcast/multicast service (MBMS) server, which is able to broadcast or multicast information to multiple participants over a geographical area. The server can provide an entry point for multicast/broadcast transmissions of a content provider providing broadcast or multicast data to communication devices situated in a geographical area served by the server.
When providing a point-to-multipoint service with multiple transmitting devices the range of the radio transmission, the quality of reception and consequently the capacity of the communication system can be greatly improved by synchronizing the transmissions from the plurality of transmitting devices. If all transmitting devices belonging to the same broadcast/multicast service area transmit the same signal at the same time, the receiving communication device can constructively use all of these transmissions and consequently sees only one very powerful transmitting device. In the art such a network is known as a single frequency network (SFN). This technique may be used in applications such as Digital Video Broadcasting for Handheld (DVB-H).
Even though it is possible to synchronise the transmitting devices for point-to-multipoint communications, it can be very costly to synchronize all transmitting devices in the area for point-to-point communications. Therefore current communication systems for point-to-point communications typically operate asynchronously. If point-to-multipoint transmission that is synchronized over a large area is combined with asynchronous point-to-point transmission, the result is a situation where the relative timing of the point-to-multipoint and point-to-point communications is variable and cannot be easily coordinated by the network or networks carrying the traffic.
End users may expect to be able to receive point-to-point i.e. unicast services such as voice calls simultaneously with multicast/broadcast service reception. When multicast/broadcast services are provided on a separate carrier frequency from the unicast service, the communication device would therefore have to be either provided with multiple parallel receivers to be able to receive from multicast/broadcast and unicast carrier frequencies at the same time, or the services would have to be provided in a time-multiplexed manner such that the multicast/broadcast and unicast transmissions for a single mobile device would not overlap in time. This also requires that the mobile device can switch between the two carriers.
The random overlapping of unicast and multicast/broadcast services on separate frequencies may cause some problems due to the above reasons. Sharing of receiver hardware in a wireless communication device may not be possible. Instead, two separate receivers may be needed. This impacts the cost, power consumption and size of the wireless communication devices. Also, the transmitter of the wireless communication device may interfere with the receivers thereof, and filtering may be needed to separate these. To enable an efficient filtering solution with reasonable cost and size in a wireless communication device, enough guard band between the uplink and downlink communications needs to be reserved.
The guard band may become a problem with spectrum allocations, where potentially available multicast/broadcast spectrum is close to a point-to-point transmitter bandwidth of an end-user communications device. The above mentioned DVB-H is an example of a communications system, where the available frequency band has to be within certain limits, more particularly to be below 750 MHz, because otherwise it would get too close to the transmitter bandwidth 900 MHz of the GSM (Global System for Mobile communications). DVB-H and GSM are uncoordinated and therefore a time-multiplexed solution has not been considered as feasible. Because of this a potential DVB-H band between 750-862 MHz cannot be used. Considering the ever increasing demand for bandwidth capacity, this can be considered as inefficient use of resources.
Carrying the unicast and multicast/broadcast communications on a single carrier frequency in a time-multiplexed transmission solves the problem of parallel receiver hardware in a wireless communication device. However, this approach limits the bandwidth available for unicast and multicast/broadcast services because they have to share the same frequency band. If unicast and multicast/broadcast frames are sent on the same carrier, and the network is operated as a single frequency network, all devices that provide shared unicast and multicast-broadcast data have to be synchronized. This has a negative impact on the cost of the network deployment. If one wants to expand the multicast/broadcast service offering, multiple parallel mixed carriers may be required, this further complicating the overall control of the system, in particular because the unicast services may need to be provided on the same carrier. Also, for example if an end user has an ongoing speech call and the user changes the TV channel to a channel which is provided on another mixed carrier, the speech call has to be handed over together with the multicast/broadcast session to be able to continue both services on the same mixed carrier.
On the other hand, if the unicast and multicast/broadcast carriers are separated, timing of the carriers becomes an issue. This is so because in a multicast/broadcast single frequency network timing is the same in the entire network. TV-channels within the multicast/broadcast carrier are time-multiplexed and each channel is sent only once in its respective frame position. The timing of the unicast carrier is, however, different in each base station and also for each user. The multicast/broadcast transmission cannot purposefully create any gaps for the unicast carrier, because each user would need them at a different time. The unicasting base station entity does not have any knowledge of what a wireless communication device may be receiving on the multicast/broadcast carrier. Neither does the unicasting base station know how that transmission is timed. The unicasting base station serving the wireless communication device may not even be involved in the multicast/broadcast transmission at all. This may be so because all base station sites of a single frequency network are not necessarily needed to create continuous multicast/broadcast coverage, due to the improved range of the single frequency network.
It is noted that these issues are not limited to any particular communication environment, but may occur in any appropriate communication system.